The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with gradient coils for use in magnetic resonance imaging, and will be described with particular reference thereto. However, it is to be appreciated that the present invention also finds application in conjunction with other applications which generate gradient magnetic fields.
Gradient coil assemblies are commonly pulsed with electrical current pulses to produce magnetic gradients across the main magnetic field in the vicinity of an imaging region within a magnetic resonance imaging system. Previously methods for production for magnetic gradients in magnetic resonance imaging systems consisted of winding discrete coils in a bunched or distributed fashion on an electrically insulating hollow cylindrical former and driving the coils with a current source of limited voltage.
Conventional bunched coil designs include the Maxwell and the Modified Maxwell Pair for z-gradient production, and the Golay or Modified Golay (multi-arc) Saddle Coils for x and/or y-gradient production. Typically, these methods consisted of iteratively placing coil loops or arcs on the cylindrical former until the desired gradient strength, gradient uniformity, and inductance (related to stored energy) were achieved. These previous designs were generally developed in a "forward approach" whereby a set of initial coil positions were defined (i.e., the initial coil distribution), the fields and the inductance/energy calculated, and if not within particular design parameters, the coil positions would be shifted (statistically or otherwise) and results re-evaluated. The iterative procedure continued until a suitable design was obtained.
More recent methods of generating magnetic fields in magnetic resonance imaging systems utilize an "inverse approach" method. In the inverse approach method, the gradient magnetic field is forced to match predetermined values at specified spatial locations inside the imaging volume and a continuous current density is calculated which is capable of producing such a field. The inverse approach method assumes that the primary gradient coil has finite dimensions while those of the secondary or shield coil are left unrestricted (infinite). After the generation of continuous current distributions for both the primary and the shield coils, an apodization algorithm is performed on the continuous current density of the shield coil in order to restrain it to desirable dimensions. Following the modification of the shielding coil's continuous current, the Stream Function technique is employed in order to obtain discrete current patterns for both coils. Application of the Biot-Savart law to the discrete current pattern ensures that the discretization procedure was proper. This approach created generally more energy efficient gradient coil assemblies with higher gradient strengths and faster slew rates as compared to the forward approach method.
One particular prior art approach is described in U.S. Pat. No. 5,296,810 to Morich. Morich describes a cylindrically shaped shielded gradient coil assembly for magnetic resonance applications. Morich uses the inverse approach method of designing gradient coil assemblies where the primary coil has a finite length while the length of the shielding coil is considered infinite. This configuration generates coils with high gradient strengths and slew rates, while at the same time reduces the eddy current effects when the length of the shield coil is substantially longer (20% or more) than the length of the primary coil. In order to restrain the current of the shielding coil within desired dimensional boundaries, apodization techniques (e.g., guassian apodization) are employed. In this manner, the overall length of the shielding coil is approximately 20% longer than the total length of the primary gradient coil. According to this invention, there is a one-to-one correspondence between the number of primary coils and secondary coil sets. Thus, the number of primary and secondary sets on the radial build up is always identical.
Another prior art shielded gradient coil design, based on the inverse approach method, is described in U.S. Pat. No. 5,132,618 to Sugimoto. In this design, both the primary and the shielded coil lengths were assumed infinite and continuous current densities for both the primary and the secondary coil are modeled based on this assumption. In order to restrain the current densities on both the primary and secondary coils, truncation is again employed. Although the outcome of this method is similar to that of the Morich patent discussed earlier, the additional truncation of the primary coil's current in this case introduces increased levels of eddy current effects inside the imaging region. According to this invention, there is a one-to-one correspondence between the number of primary coils and secondary coil sets. Thus, the number of primary and secondary sets on the radial build up is always identical.
U.S. Pat. No. 4,794,338, to Roemer, et al. discloses an alternative approach of designing a shielded gradient coil set based on the forward approach method. The outcome is a shielded gradient coil set with a moderate to low efficiency rate in terms of gradient strength and slew rate. Further, there is no precondition to the method of controlling the eddy current effects inside the imaging region. According to this invention, there is a one-to-one correspondence between the number of primary coils and secondary coil sets. Thus, the number of primary and secondary sets on the radial build up is always identical.
As described in the prior art references above, the design of a conventional shielded gradient coil set has been generally based on the assumption that for each primary coil there is a corresponding screening coil which shields the fringe field of the primary coil in the vicinity of the magnet dewar and magnet shields. According to this methodology, a double duty shielded gradient coil set must include two sets of primary coils (x, y, z) and two sets of secondary coils (x, y, z). Specifically, each screening coil can only be related to one primary coil, and thus, in the case of a double duty gradient coil set, six primary and six secondary coils are necessary.
U.S. Pat. No. 5,736,858 to Katznelson, et al. discloses such a double duty coil geometry having two sets of primary coils shielded by two sets of shielding coils, where each primary coil has a one-to-one correspondence with a shielding coil. That is, two different shielding coils are employed to screen two different primary coils with different lengths and imaging volumes. Therefore, the number of primary and secondary coils in the gradient build-up are always identical. The Katznelson coil geometry was designed using the inverse approach method where trade-offs between linearity and coil performance are taken into account. In addition, the imaging volume, and the performance levels of the two gradient coil sets are different. Furthermore, both primary gradient coil sets have different lengths. Specifically, the primary and shield gradient coil combination with the better linearity, lower efficiency, and larger imaging volume, is longer lengthwise than the primary and shield coil combination that has higher efficiency but lower field quality and smaller imaging volume. Thus, the complete gradient coil configuration consists of twelve gradient coils (six primary coil groups and six screening coil groups) configured as two separate modular shielded gradient coil entities, each consisting of three primary and three shielded coils
The present invention contemplates a new and improved shielded gradient coil assembly shielded gradient coil assembly having two primary gradient coil sets and a common screening coil set which overcomes the above-referenced problems and others.